The present invention relates to a magnetic recording medium, non-magnetic acicular black iron-based composite particles and a process for producing the non-magnetic acicular black iron-based composite particles, and more particularly, to a magnetic recording medium having a smooth surface, a lower light transmittance, a lower surface resistively value, a low friction coefficient and an excellent running durability; non-magnetic acicular black iron-based composite particles having an excellent dispersibility in vehicle due to less amount of carbon black desorbed or fallen-off from the surfaces thereof, a more excellent blackness, a lower volume resistivity value and a well-controlled myristic acid absorption; a process for producing the non-magnetic acicular black iron-based composite particles; and a non-magnetic substrate for magnetic recording media having a smooth surface and a lower surface resistivity value.
With a development of miniaturized and lightweight video or audio magnetic recording and reproducing apparatuses for long-time recording, magnetic recording media such as a magnetic tape and magnetic disk have been increasingly and strongly required to have a higher performance, namely, a higher recording density, higher output characteristic, in particular, an improved frequency characteristic and a lower noise level.
Video tapes have recently been required more and more to have a higher picture quality, and the frequencies of carrier signals recorded in recent video tapes are higher than those recorded in conventional video tapes. In other words, the signals in the short-wave region have come to be used, and as a result, the magnetization depth from the surface of a magnetic tape has come to be remarkably small.
With respect to short wavelength signals, a reduction in the thickness of a magnetic recording layer is also strongly demanded in order to improve the high output characteristics, especially, the S/N ratio of a magnetic recording medium. This fact is described, for example, on page 312 of
Development of Magnetic Materials and Technique for High Dispersion of Magnetic Powder, published by Sogo Gijutsu Center Co., Ltd. (1982), xe2x80x9c . . . the conditions for high-density recording in a coated-layer type tape are that the noise level is low with respect to signals having a short wavelength and that the high output characteristics are maintained. To satisfy these conditions, it is necessary that the tape has large coercive force Hc and residual magnetization Br, . . . and the coating film has a smaller thickness, . . . xe2x80x9d.
Development of a thinner film for a magnetic recording layer has caused some problems.
Firstly, it is necessary to make a magnetic recording layer smooth and to eliminate the non-uniformity of thickness. As well known, in order to obtain a smooth magnetic recording layer having a uniform thickness, the surface of the base film must also be smooth. This fact is described on pages 180 and 181 of Materials for Synthetic Technologyxe2x80x94Causes of Friction and Abrasion of Magnetic Tape and Head Running System and Measures for Solving the Problem (hereinunder referred to as xe2x80x9cMaterials for Synthetic Technologyxe2x80x9d (1987), published by the Publishing Department of Technology Information Center, xe2x80x9c . . . the surface roughness of a hardened magnetic coating film depends on the surface roughness of the base film (back surface roughness) so largely as to be approximately proportional, . . . , since the magnetic coating film is formed on the base film, the more smooth the surface of the base film is, the more uniform and larger head output is obtained and the more the S/N ratio is improved.xe2x80x9d
Secondly, there has been caused a problem in the strength of a base film with a tendency of the reduction in the thickness of the base film in response to the demand for a thinner magnetic coating film. This fact is described, for example, on page 77 of the above-described Development of Magnetic Materials and Technique for High Dispersion of Magnetic Powder, xe2x80x9c . . . Higher recording density is a large problem assigned t the present magnetic tape. This is important in order to shorten the length of the tape so as to miniaturize the size of a cassette and to enable long-time recording. For this purpose, it is necessary to reduce the thickness of a base film . . . With the tendency of reduction in the film thickness, the stiffness of the tape also reduces to such an extent as to make smooth travel in a recorder difficult. Therefore, improvement of the stiffness of a video tape both in the machine direction and in the transverse direction is now strongly demanded. . . . xe2x80x9d
The end portion of a magnetic recording medium such as a magnetic tape, especially, a video tape is judged by detecting a portion of the magnetic recording medium at which the light transmittance is large by a video deck. If the light transmittance of the whole part of a magnetic recording layer is made large by the production of a thinner magnetic recording medium or the ultrafine magnetic particles dispersed in the magnetic recording layer, it is difficult to detect the portion having a large light transmittance by a video deck. For reducing the light transmittance of the whole part of a magnetic recording layer, carbon black fine particles or the like is added to the magnetic recording layer. It is, therefore, essential to add carbon black or the like to a magnetic recording layer in the present video tapes.
However, addition of a large amount of non-magnetic particles such as carbon black fine particles impairs not only the enhancement of the magnetic recording density but also the development of a thinner recording layer. In order to reduce the magnetization depth from the surface of the magnetic tape and to produce a thinner magnetic recording layer, it is strongly demanded to reduce, as much as possible, the quantity of non-magnetic particles such as carbon black fine particles which are added to a magnetic recording layer.
Consequently, it has been strongly demanded to provide a magnetic recording medium capable of exhibiting a low light transmittance even when the amount of carbon black fine particles added to a magnetic recording layer thereof is reduced as low as possible. From this viewpoint, it has been strongly required to essentially improve properties of a substrate therefor.
Further, in order to reduce not only the above-mentioned light transmittance of the magnetic recording medium but also an electric resistance thereof, there has been hitherto proposed a method of adding carbon black fine particles to the magnetic recording layer.
The conventional magnetic recording medium to which carbon black fine particles are added, is described in detail below.
When a magnetic recording medium has a high surface resistivity, an electrostatic charge thereon tends to be increased, so that cut chips of the magnetic recording medium and dirt or dusts are attached onto the surface of the magnetic recording medium upon production or use thereof, and as a result, such a problem that the number of xe2x80x9cdrop-outxe2x80x9d becomes increased, is caused.
In order to decrease the surface resistivity of the magnetic recording medium to about 108 xcexa9/cm2, a conductive compound such as carbon black fine particles has been ordinarily added to the magnetic recording layer in an amount of not less than about 5 parts by weight based on 100 parts of magnetic particles contained in the magnetic recording layer.
However, when the amount of carbon black fine particles added or the like which cannot contribute to improvement in magnetic properties of the magnetic recording layer, is increased, the magnetic recording medium has been deteriorated in electromagnetic performance as described above, resulting in inhibiting the magnetic recording layer from being thinned.
It has been required to further enhance the performance of magnetic recording media. The magnetic recording media have been strongly required to exhibit not only high-density recording property but also improved physical properties such as running property.
A good running property of the magnetic recording media can be usually assured by adding to a magnetic recording layer ordinarily formed as an upper layer of the magnetic recording medium, a fatty acid such as myristic acid and stearic acid (hereinafter referred to merely as xe2x80x9cmyristic acidxe2x80x9d) in an amount of usually about 0.5 to about 5% by weight based on the weight of magnetic particles used in the magnetic recording layer. The thus added myristic acid is controlled so as to be gradually oozed from the magnetic recording layer, thereby rendering the surface thereof more slippery.
When the amount of the myristic acid oozed onto the surface of the magnetic recording layer is too small, it is difficult to impart a good running property to the magnetic recording media. On the contrary, when a large amount of the myristic acid is added to the magnetic recording layer in order to increase the amount of the myristic acid oozed onto the surface thereof, the myristic acid is preferentially absorbed onto the surfaces of magnetic particles dispersed in the magnetic recording layer, thereby inhibiting resins from being absorbed onto the surfaces of the magnetic particles. As a result, it becomes difficult to disperse the magnetic particles in vehicle. Also, the increased content of the myristic acid as a non-magnetic component tends to cause deterioration in magnetic properties of the obtained magnetic recording media. Further, since the myristic acid acts as a plasticizer, there arise problems such as deteriorated strength of the magnetic recording media.
With the recent tendency toward reduction in thickness of the magnetic recording layer, an absolute amount of myristic acid being addable thereto is decreased. Also, in order to achieve high-density recording on magnetic recording media, it has been required that magnetic particles used therein are much finer, resulting in increase in BET specific surface area thereof. The increment in BET specific surface area of the magnetic particles increases an amount of the myristic acid absorbed onto the surfaces thereof. As a result, it becomes more and more difficult to assure a good running property of the magnetic recording layer, by controlling the amount of the myristic acid oozed onto the surface of the magnetic recording layer only by a amount of myristic acid added to the magnetic recording layer.
In Japanese Patent Application Laid-Open (KOKAI) No. 5-182178 (1993), it is described that xe2x80x9cin the present invention . . . , inorganic particles and fatty acid contained in the non-magnetic layer are prevented from being interacting with each other to control respective contents of the fatty acid in the non-magnetic layer and the magnetic recording layer, thereby improving a running durability of the magnetic recording layer . . . xe2x80x9d. As understood from the above description, it has been strongly required to assure a good running property of magnetic recording media by appropriately controlling the amount of myristic acid oozed onto the surface of the magnetic recording layer, by means of both the magnetic recording layer and the non-magnetic undercoat layer having a thickness two or more times that of the magnetic recording layer.
With the reduction in thickness of the magnetic recording layer or the base film therefor, various attempts have been conducted in order to improve properties of a substrate on which the magnetic recording layer is formed, thereby enhancing a surface smoothness and a strength of the magnetic recording medium. In this regard, there has been proposed a method of forming on a base film, at least one undercoat layer composed of a binder resin and non-magnetic particles dispersed in the binder resin and containing iron as a main component, such as acicular hematite particles or acicular iron oxide hydroxide particles (hereinafter referred to merely as xe2x80x9cnon-magnetic undercoat layerxe2x80x9d), and such a method has been already put into practice (Japanese Patent Publication (KOKOKU) No. 6-93297(1994), Japanese Patent Application Laid-open (KOKAI) Nos. 62-159338(1987), 63-187418(1988), 4-167225(1992), 4-325915(1992), 5-73882(1993), 5-182177(1993), 5-347017(1993) and 6-60362(1994), and the like).
It is known to use as the non-magnetic particles for non-magnetic undercoat layer, non-magnetic particles which surfaces are coated with hydroxides of aluminum, oxides of aluminum, hydroxides of silicon, oxides of silicon or a mixture thereof in order to improve the dispersibility thereof in vehicles, thereby enhancing a surface smoothness and a strength of the obtained non-magnetic substrate (Japanese Patent Nos. 2,571,350 and 2,582,051, and Japanese Patent Application Laid-open (KOKAI) Nos. 6-60362(1994), 9-22524(1997) and 9-27117(1997) or the like).
Also, it is known that in order to decrease an amount of carbon black fine particles added to the magnetic recording layer and reduce a light transmittance of the magnetic recording medium, blackish brown acicular hematite particles or blackish brown acicular iron oxide hydroxide particles are used as the non-magnetic particles for non-magnetic undercoat layer (Japanese Patent Application Laid-open (KOKAI) Nos. 7-66020(1995), 8-259237(1996) and 9-167333(1997)).
It is also known to use non-magnetic acicular particles such as acicular hematite particles and acicular iron oxide hydroxide particles on which surfaces carbon black is adhered in an amount of 1 to 20 parts by weight based on 100 parts by weight of the non-magnetic acicular particles (European Patent No. 0924690 A).
Further, it is known that in order to reduce an electric resistance of the magnetic recording medium, a mixture of non-magnetic iron oxide particles and carbon black fine particles is used as the non-magnetic particles for non-magnetic undercoat layer (Japanese Patent Application Laid-open (KOKAI) Nos. 1-213822(1989), 1-300419(1989), 6-236542(1994) and 9-297911(1997) or the like).
At present, it has been most demanded to provide magnetic recording media having a smooth surface, a lower light transmittance, a lower surface resistivity value and an excellent running property. However, non-magnetic particles for non-magnetic undercoat layer which can provide such magnetic recording media satisfying all of the above requirements, have not been obtained.
Namely, the magnetic recording media produced by using the above conventional acicular hematite particles or acicular iron oxide hydroxide particles whose surfaces are coated with a hydroxide of aluminum, an oxide of aluminum, a hydroxide of silicon or an oxide or silicon, as non-magnetic particles for non-magnetic undercoat layer, have a smooth surface, but fail to reduce a light transmittance thereof because the non-magnetic particles exhibit from dark-red to yellowish brown color. Further, these magnetic recording media have a surface resistivity value as high as about 1013 xcexa9/cm2, and a friction coefficient (as an index of the running property) as poor as about 0.33.
Further, in the case of the above-mentioned substrate using as non-magnetic particles for non-magnetic undercoat layer, blackish brown acicular hematite particles or blackish brown acicular iron oxide hydroxide particles, the obtained non-magnetic undercoat layer can show a higher degree of blackness as compared to those using the dark red acicular hematite particles or the yellowish brown acicular iron oxide hydroxide particles, so that it becomes possible to reduce a light transmittance of the substrate. However, the reduction of light transmittance is still insufficient. In addition, the surface resistivity of the non-magnetic substrate is as large as about 1012 xcexa9/cm2, and a friction coefficient (as an index of the running property) as poor as about 0.33.
In the case of a magnetic recording medium produced by using non-magnetic particles for a non-magnetic undercoat layer described in European Patent No. 0924690 A, due to an excellent blackness and conductivity of carbon black, the light transmittance and surface resistivity value of the magnetic recording medium are improved. Especially, most excellent linear absorption as an index of the light transmittance thereof is 2.71 xcexcmxe2x88x921 and the surface resistivity value thereof is 1.2xc3x97108 xcexa9/cm2. As shown in Comparative example described later, the friction coefficient as an index of the running property is as poor as about 0.32.
In the case of the non-magnetic particles described in the above-mentioned Japanese Patent Application Laid-open (KOKAI) Nos. 1-213822(1989), 1-300419(1989) and 9-297911(1997), there have been used the mixture of non-magnetic iron oxide particles and carbon black fine particles in which the carbon black fine particles are added in an amount of not less than 25 parts by weight based on 100 parts by weight of the non-magnetic iron oxide particles. Therefore, due to the fact that the carbon black fine particles which show the largest degree of blackness among various black pigments, are used in such a large amount, it is possible to obtain a non-magnetic substrate having a low light transmittance and a low electric resistance. However, it has been difficult to disperse the carbon black fine particles in vehicles, since the carbon black fine particles are fine particles having such a small average diameter as about 0.002 to about 0.05 xcexcm, a large specific surface area and a deteriorated solvent wettability, thereby failing to obtain a non-magnetic substrate having a smooth surface. Also, the friction coefficient (as an index of the running property) is as poor as about 0.31. In addition, the carbon black fine particles have a bulk density as low as about 0.1 g/cm3 and, therefore, the carbon black fine particles are bulky particles, resulting in deteriorated handing property and workability. Further, with respect to the carbon black fine particles, many problems concerning safety or hygiene such as carcinogenesis have been pointed out.
Thus, as the amount of the carbon black fine particles added to the non-magnetic substrate is increased, the light transmittance of the obtained non-magnetic substrate tends to become small. However, when the carbon black fine particles are used in a large amount, it becomes more difficult to disperse the particles in vehicles, resulting in deteriorated workability. Further, the use of a large amount of the carbon black fine particles are disadvantageous in view of safety and hygiene.
In the above-mentioned Japanese Patent Application Laid-open (KOKAI) No. 6-236542(1994), there have been described such non-magnetic particles mixture of non-magnetic iron oxide particles and carbon black fine particles having a specific structure in which the carbon black fine particles are used in an amount of 1 to 17.6 parts by weight based on 100 parts by weight of the non-magnetic iron oxide particles. By using such specific carbon black fine particles having a high conductivity, the electric resistance of the obtained non-magnetic substrate can be reduced even at small carbon black fine particles content. However, since the amount of carbon black fine particles used is small, it is difficult to reduce a light transmittance of the non-magnetic substrate.
As a result of the present inventors"" earnest studies for solving the above conventional problems, it has been found that by using as non-magnetic particles non-magnetic acicular black iron-based composite particles which comprise acicular hematite particles or acicular iron oxide hydroxide particles as core particles; a coating layer formed on the surface of each core particle, at least one organosilicon compound; and a carbon black coat formed on at least a part of the surface of the coating layer in an amount of 21 to 50 parts by weight based on 100 parts by weight of the acicular hematite particles or acicular iron oxide hydroxide particles, and which have an average major axial diameter of 0.011 to 0.35 xcexcm and a myristic acid absorption of 0.01 to 0.3 mg/m2, the obtained magnetic recording medium can exhibit a smooth surface, a lower light transmittance, a lower surface resistivity value, a low friction coefficient and an excellent running property. The present invention has been attained on the basis of this finding.
An object of the present invention is to provide a magnetic recording medium exhibiting a smooth surface, a lower light transmittance, a lower surface resistivity value, a low friction coefficient and an excellent running property.
Another object of the present invention is to provide non-magnetic acicular black iron-based composite particles exhibiting an excellent dispersibility in vehicle due to less amount of carbon black desorbed or fallen-off from the surface of each composite particle, a more excellent blackness, a lower volume resistivity value and a well-controlled myristic acid absorption, and a process for producing the composite particles.
A further object of the present invention is to provide a non-magnetic substrate for magnetic recording media, having a smooth surface and a lower surface resistively value.
To accomplish the aims, in a first aspect of the present invention, there is provided a magnetic recording medium comprising:
a non-magnetic base film;
a non-magnetic undercoat layer formed on the non-magnetic base film, comprising a binder resin and non-magnetic acicular black iron-based composite particles; and
a magnetic coating film formed on the non-magnetic undercoat layer, comprising a binder resin and magnetic particles,
the said non-magnetic acicular black iron-based composite particles having an average major axis diameter of usually 0.011 to 0.35 xcexcm, comprising:
acicular hematite particles or acicular iron oxide hydroxide particles;
a coating layer formed on the surface of the acicular hematite particle or acicular iron oxide hydroxide particle, comprising at least one organosilicon compound selected from the group consisting of:
(1) organosilane compounds obtained from an alkoxysilane compounds, and
(2) polysiloxanes or modified polysiloxanes; and
a single carbon black coat formed on at least a part of the coating layer comprising the organosilicon compound, in an amount of 21 to 50 parts by weight based on 100 parts by weight of the acicular hematite particles or acicular iron oxide hydroxide particles.
In a second aspect of the present invention, there is provided a magnetic recording medium comprising:
a non-magnetic base film;
a non-magnetic undercoat layer formed on the non-magnetic base film, comprising a binder resin and non-magnetic acicular black iron-based composite particles; and
a magnetic coating film formed on the non-magnetic undercoat layer, comprising a binder resin and magnetic particles,
the said non-magnetic acicular black iron-based composite particles having an average major axis diameter of usually 0.011 to 0.35 xcexcm, comprising:
acicular hematite particles or acicular iron oxide hydroxide particles;
a coat formed on at least a part of the surface of the acicular hematite particles or acicular iron oxide hydroxide particles, comprising at least one compound selected from the group consisting of hydroxides of aluminum, oxides of aluminum, hydroxides of silicon and oxides of silicon;
a coating layer formed on the said coat formed on the surface of the acicular hematite particle or acicular iron oxide hydroxide particle, comprising at least one organosilicon compound selected from the group consisting of:
(1) organosilane compounds obtained from an alkoxysilane compounds, and
(2) polysiloxanes or modified polysiloxanes; and
a single carbon black coat formed on at least a part of the coating layer comprising the organosilicon compound, in an amount of 21 to 50 parts by weight based on 100 parts by weight of the acicular hematite particles or acicular iron oxide hydroxide particles.
In a third aspect of the present invention, there is provided a non-magnetic substrate comprising:
a non-magnetic base film; and
a non-magnetic undercoat layer formed on the non-magnetic base film, comprising a binder resin and non-magnetic acicular black iron-based composite particles,
the said non-magnetic acicular black iron-based composite particles having an average major axis diameter of usually 0.011 to 0.35 xcexcm, comprising:
acicular hematite particles or acicular iron oxide hydroxide particles;
a coating layer formed on the surface of the acicular hematite particle or acicular iron oxide hydroxide particle, comprising at least one organosilicon compound selected from the group consisting of:
(1) organosilane compounds obtained from an alkoxysilane compounds, and
(2) polysiloxanes or modified polysiloxanes; and
a single carbon black coat formed on at least a part of the coating layer comprising the organosilicon compound, in an amount of 21 to 50 parts by weight based on 100 parts by weight of the acicular hematite particles or acicular iron oxide hydroxide particles.
In a fourth aspect of the present invention, there is provided a non-magnetic substrate comprising:
a non-magnetic base film; and
a non-magnetic undercoat layer formed on the non-magnetic base film, comprising a binder resin and non-magnetic acicular black iron-based composite particles,
the said non-magnetic acicular black iron-based composite particles having an average major axis diameter of usually 0.011 to 0.35 xcexcm comprising:
acicular hematite particles or acicular iron oxide hydroxide particles;
a coat formed on at least a part of the surface of the acicular hematite particles or acicular iron oxide hydroxide particles, comprising at least one compound selected from the group consisting of hydroxides of aluminum, oxides of aluminum, hydroxides of silicon and oxides of silicon;
a coating layer formed on the said coat formed on the surface of the said acicular hematite particle or acicular iron oxide hydroxide particle, comprising at least one organosilicon compound selected from the group consisting of:
(1) organosilane compounds obtained from an alkoxysilane compounds, and
(2) polysiloxanes or modified polysiloxanes; and
a single carbon black coat formed on at least a part of the coating layer comprising the organosilicon compound, in an amount of 21 to 50 parts by weight based on 100 parts by weight of the acicular hematite particles or acicular iron oxide hydroxide particles.
In a fifth aspect of the present invention, there are provided non-magnetic acicular black iron-based composite particles having an average major axis diameter of usually 0.011 to 0.35 xcexcm, comprising:
acicular hematite particles or acicular iron oxide hydroxide particles;
a coating layer formed on the surface of the acicular hematite particle or acicular iron oxide hydroxide particle, comprising at least one organosilicon compound selected from the group consisting of:
(1) organosilane compounds obtained from an alkoxysilane compounds, and
(2) polysiloxanes or modified polysiloxanes; and
a single carbon black coat formed on at least a part of the coating layer comprising the organosilicon compound, in an amount of 21 to 50 parts by weight based on 100 parts by weight of the acicular hematite particles or acicular iron oxide hydroxide particles.
In a sixth aspect of the present invention, there are provided non-magnetic acicular black iron-based composite particles having an average major axis diameter of usually 0.011 to 0.35 xcexcm, comprising:
acicular hematite particles or acicular iron oxide hydroxide particles;
a coat formed on at least a part of the surface of the acicular hematite particles or acicular iron oxide hydroxide particles, comprising at least one compound selected from the group consisting of hydroxides of aluminum, oxides of aluminum, hydroxides of silicon and oxides of silicon;
a coating layer formed on the said coat formed on the surface of the acicular hematite particle or acicular iron oxide hydroxide particle, comprising at least one organosilicon compound selected from the group consisting of:
(1) organosilane compounds obtained from an alkoxysilane compounds, and
(2) polysiloxanes or modified polysiloxanes; and
a single carbon black coat formed on at least a part of the coating layer comprising the organosilicon compound, in an amount of 21 to 50 parts by weight based on 100 parts by weight of the acicular hematite particles or acicular iron oxide hydroxide particles.